The present invention relates to a driver circuit for brushless DC motor and in particlular to a current control type driver circuit suitable to a chopping type driver circuit for controlling the rotation of a motor with an accelerating torque at lagging of the motor speed and a decelerating torque at leading of the motor speed.
In a chopping type driver circuit for brushless motor such as disclosed in, for example, Japanese Patent Unexamined Publication No. 57-145586, the speed error signal is detected and the current flowing through the winding of the motor is switched by switching elements on the basis of the detected speed error signal.
Denoting the input voltage of the motor as E, the speed electromotive force which depends on the rotation of the motor as Ea, the resistance of the stator winding as Ra, the current flowing into the stator as Ia, and proportionality constant as K, the acceleration control torque T.sub.F and the deceleration control torque T.sub.B during the motor rotation can be represented as, EQU T.sub.F =kIa=k(E-Ea)/Ra EQU T.sub.B =kI'a=k(-E-Ea)/Ra=K'(E+Ea)/Ra
where K'=-K.
That is to say, the acceleration control torque during the rotation is different from the deceleration control torque during the rotation because of the influence of the speed electromotive force. On the basis of the speed error signal, therefore, the number of rotations during rotation is controlled with different torques. This results in a drawback that the number of rotations cannot be controlled smoothly.